Hello
I am interested in selling my rackets I bought from someone here on TW, but I never sold anything online in my life. I have read the guidelines and have a few questions.

After someone has confirmed they wanted to buy it, what do I do from their with paypal and all? And concerning about shipping, can I just go to any UPS store and ask for boxes that will fit tennis rackets, and it's free? And since this is my first online trading/selling and the other people has some feedback how do I get him/her to trust me?

This is mostly covered in the Sticky... (did you read it?)

A) Get a paypal account if you haven't already. Jump through the hoops and link it to your bank account, or whatever you feel comfortable with.
B) Go to USPS, it's cheaper (generally) and the boxes are free. (covered in sticky).
C) Sometimes new traders have to ship first to established traders when they start out. Work that part out with the party you're trading with.